1. Field of the Invention
This invention relates to an apparatus for precisely working aspheric surfaces having mirror surfaces of metals, ceramic materials, glass materials, and so on.
2. Description of the Prior Art
High precision mirror surface processing of a mold for a plastic lens is conventionally performed by cutting or grinding with a computerized numerical control (CNC) lathe or curve generator and after which polishing is performed by a polishing machine or by hand works. Another conventional method for processing a mold for a plastic lens is by processing the material to be formed as the mold by single point diamond turning.
On the other hand, hard metals or ceramics are used as a mold for a glass lens. The mold made of hard and brittle materials such as ceramics or glasses is conventionally worked by moving a diamond grinder, which rotates at a high speed, along a desired working shape to simultaneously work the material into a desired configuration with a mirror surface.
When a free curved surface of a mold is worked by CNC milling or by CNC electrical discharge machining, cutting, grinding or electrical discharge machining is performed, control is provided with CNC positioning in accordance with predetermined working data. The curved surface worked by those methods has a surface roughness of the order of several micron meters and a mirror surface is not obtained. The process tolerance of the processed surface is on the order of several micron meters which is defined by the accuracy of the processing machines. As a result, a finishing process is required to obtain a mirror surface. Usually, the finishing process is performed by polishing the workpieces by hand with observing the processed shape. Instead of the hand polishing, automatic polishing methods for curved surfaces are sometimes introduced.
However, the conventional methods have several defects. When the cutting or grinding is performed, the worked surface has a surface roughness on the order of a micron meter and the shape accuracy is also in the range of a micron meter. Therefore, it is necessary to polish the workpiece to obtain an accuracy on the order of sub-micron meter by an operator's hand polishing which requires operator's skill and inefficiency. The electrical discharge machining has the same defects.
The diamond cutting and diamond grinding can obtain the surface roughness and shape accuracy on the order of a sub-micron meter which is able to realize processing a mirror surface. However, these methods require a finishing process to eliminate trace marks of the tool. As described above, the conventional methods require a preworking process for obtaining a desired shape and a finishing process for obtaining a mirror surface. Furthermore, the grindstone and the rotating mechanism thereof in a grinding process and the polisher in polishing process are difficult to miniaturize in terms of their sizes, and it is difficult to grind or polish a minute region on the curved surfaces.